When pulled as tight as possible, the rudder hangs 2-3 inches below top. I can easily push the rudder up by hand from here. Is there an adjustment I can make so that pulling the rudder cord raises the rudder all the way?

Not really sure what could cause this - there must be a burr, or the spring is out of alignment - in any case, this would fall under your warranty - so just visit your nearest dealer can get the rudder fixed or replaced.

As far as the warranty, I can't recall if there is an official process. If you have the original bill of sale that will be fine. The serial number can be used to determine the date of manufacture, but in the case of a boat like mine, I bought a 2011 Oasis in 2012, so the warranty doesn't start until the date of purchase. If you have a similar situation, the bill of sale might be more important than just the serial number, etc.